The present invention relates to a magnetic recording medium, particularly, to improving the surface properties of a magnetic tape.
In recent years, it has been strongly desired to form magnetic layers of excellent surface smoothness on base materials in the magnetic tape, especially the video tape, art. As examples of processes for improving the surface roughness of the magnetic layer, there is known (1) the calender process in which the magnetic layer is heated and pressed between a metal roll and an elastic roll after it has been formed on a base material, subjected to orientation and then dried (as described in U.S. Pat. Nos. 2,688,567, 2,998,325 and 3,783,023, German Patent application (OLS) No. 2,405,222, Japanese Patent Applications (OPI) Nos. 53631/74, 10337/75, 99506/75, 92606/76, 102049/76 and 10340/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), Japanese Patent Publication No. 17404/77, etc.); (2) the smoothing process in which the surface of the magnetic layer is smoothed prior to drying with a magnetic smoother, a smoothing coil, a smoothing blade, a smoothing blanket or the like (as described in Japanese Patent Publications 38802/72 and 11336/73, Japanese Patent Applications (OPI) 53631/74, 112005/75 and 77303/76, British Pat. No. 1,191,424, and the like).
However, even a magnetic tape having a magnetic layer surface smoothed by one of the above described processes suffers from various defects, e.g., the magnetic layer surface has a high coefficient of friction, is easily scraped and damaged, adheres to a tape transporting system in a video tape handler (which comprises rotating drums, capstans, tape guides, etc.) and undergoes cinching when wound around a reel, hub or the like, and so on.
Thus, improvements in surface roughness of a magnetic layer attained by the above described processings adversely affect tape transporting characteristics to a great extent.
On the other hand, methods for improving tape transporting characteristics are also known (as described in, e.g., Japanese Patent Application (OPI) No. 128683/78) in which the surface roughness of the back side of a magnetic tape is increased by providing a backing layer which contains a lubricant on the surface of the non-magnetic flexible support opposite to the surface whereon a magnetic layer is formed.
However, it turns out that with this method the magnetic tape still suffers the disadvantages that the coated backing layer is liable to be scraped by contact with parts of a tape transporting system to cause partial dropout of the backing layer, scraping of the coated material from the backing layer is accelerated by the coated material scraped from the backing layer adhering to the tape transporting system, the gap of a magnetic head is stopped up with the coated material scraped from the backing layer and so on. Further, it is very difficult to apply the backing layer to have a uniform thickness.
In accordance with the above described calender process, smoothing process, backing layer process and the like, the chroma signal to noise ratio (hereinafter abbreviated as C-S/N) and the video signal to noise ratio (hereinafter abbreviated as Y-S/N) can be improved, but magnetic tapes having all characteristics adequate for practical use cannot be obtained.
An improvement in the surface property of a magnetic tape can be attained by the use of a base material possessing an excellent surface property and, further, by appropriately selecting ingredients for the magnetic layer and by subjecting the surface of a magnetic layer to calender processing.
Also, an improvement in tape transporting characteristics can be attained by a careful selection of lubricants to be incorporated into a magnetic layer or by increasing the surface roughness of a magnetic layer or the base to a certain extent. However, in order to increase both the C-S/N and the Y-S/N ratios it is necessary to render the surface roughness of the magnetic layer as low as possible, that is to say, to render the surface of the magnetic layer as smooth as possible which is essentially incompatible with the necessity to render the surface of the magnetic layer somewhat rough to improve tape transporting characteristics.
Accordingly, a balanced improvement in both the C-S/N and Y-S/N ratios and the tape transporting characteristics using conventional methods have turned out to be difficult.